1. Field of the Invention
The present invention relates to a driving device, an exposure apparatus using the driving device, and a device manufacturing method and, more particularly, to an exposure apparatus which can execute, using an electromagnetic actuator, high-speed, high-accuracy, high-efficiency, low-heat-generation stage control and anti-vibration control.
2. Description of the Related Art
In recent years, a demand has arisen for higher productivity of semiconductor integrated circuits, such as ICs and LSIs. Along with this trend, a semiconductor exposure apparatus needs to execute a high-speed exposure process. Meanwhile, to micronize the circuit pattern, an exposure target wafer and a reticle as a master of the circuit pattern need to be aligned on the order of nm or less.
A scanning exposure apparatus synchronously scans a reticle stage which holds a reticle and a wafer stage which holds a wafer. Exposure is performed while the reticle stage and wafer stage move at constant velocities in opposite directions. In general, the projection magnification ratio of a reticle to a wafer is 4:1, and the scanning velocity ratio of a reticle stage to a wafer stage is also 4:1. To improve the productivity, these stages are required to be driven at higher scanning velocities. Along with this demand, the scanning velocity of a wafer stage reaches several hundred mm/s.
As described above, a reticle stage and a wafer stage are required to be driven with high speed and high accuracy. Therefore, an actuator, which can drive these stages with high efficiency, low heat generation, and high accuracy, becomes indispensable.
As the actuator which drives a stage at high speed, a linear motor, which takes advantage of the Lorentz force generated upon supplying a current to a magnetic field, is available. The main constituent components of the linear motor are magnets, which generate magnetic fields, and coils, which supply currents. The relationship between the coil current and the obtained Lorentz force, i.e., thrust, exhibits good linearity and controllability. However, the linear motor requires a large power to obtain a large thrust because of poor efficiency. As a result, if the actuator is arranged near the reticle or the wafer, it becomes sometimes difficult to attain an alignment accuracy on the order of nm, due to heat from the actuator.
As an actuator capable of obtaining a large thrust with high efficiency and low heat generation, i.e., a small power, one that uses the principle of an electromagnetic actuator (also called an electromagnet) is available. A general electromagnetic actuator includes coils and two members containing ferromagnetic bodies. The two members are arranged to oppose each other through a small gap. One of these members is wound with a coil. A current is supplied to the coil to generate a closed magnetic flux loop in the two members. This produces an attraction force in the gap, through which these members oppose each other, thereby obtaining a thrust. In general, the gap is set at a very narrow interval, i.e., about several tens to several hundred μm. An actuator with very high efficiency can thus be realized.
In order to realize high-accuracy exposure by suppressing vibration conducted from the installation floor of an exposure apparatus, or a disturbance acting upon driving a wafer stage, an anti-vibration device for vibration insulation or vibration suppression becomes indispensable. While a disturbance acting upon driving the wafer stage at high speed becomes larger, further micropatterning is demanded of the exposure apparatus. To improve the vibration insulation or vibration suppression performance of the anti-vibration device, an active control operation for causing an actuator to cancel the disturbance is widely employed. A linear motor or an electromagnetic actuator is used as the actuator, but the advent of an actuator capable of obtaining a large thrust with low heat generation and high accuracy is demanded.
Unfortunately, an actuator which adopts an electromagnetic actuator poses the following problems. The generated magnetic flux is proportional to the coil current and inversely proportional to the gap. The relationship between the thrust and the magnetic flux density is given by:F∝B2  (1)v(F)∝B  (2)φ=B·S  (3)φ∝I  (4)where F is the thrust, B is the magnetic flux density, φ is the magnetic flux, I is the coil current, and S is the cross-sectional area of the gap through which the members of the electromagnetic actuator oppose each other. From this relationship, it is found that the thrust exhibits a nonlinear characteristic to the coil current and gap, although high efficiency is achieved. Also, even a small change in coil current or gap causes a large variation in thrust.
A stage device using an electromagnetic actuator disclosed in Japanese Patent Laid-Open No. 2002-033270 is designed to be used such that the gap interval in the electromagnetic actuator is measured by a sensor and corrected to supply a coil current depending on the gap interval. However, this technique adopts a thrust correction method using an open loop to correct the coil current, depending on the measured gap interval, thereby matching the thrust with a predetermined one. Therefore, this technique is inappropriate as the control method for an actuator which executes high-accuracy control due to variations in characteristics of the electromagnetic actuator and coil.
In an alignment apparatus using an electromagnetic actuator disclosed in Japanese Patent Application Laid-Open No. 2002-319535, a change in magnetic flux generated in the magnetic actuator is detected by a newly arranged coil, integrated by an electrical integrator, and converted into a signal proportional to the magnetic flux. In accordance with the above equations (2) and (3), a feedback loop is formed to control the obtained signal to manipulate a coil current so as to match the thrust with a predetermined thrust F, thereby suppressing variations in characteristics of the electromagnetic actuator and coil. However, owing to the offset currents and offset voltages of the electronic components, which form an integrator and a signal amplifier, which detects a change in magnetic flux, an output from the integrator generates a lamp-shaped offset that increases along with the elapse of time even when the change in magnetic flux is zero. Finally, the output reaches the saturated voltage and becomes uncontrollable. To suppress this phenomenon, a high-pass filter is inserted in the feedback loop. However, the high-pass filter cannot make a response to a DC thrust or a thrust in a low-frequency band. For this reason, this technique is inappropriate as a control method for an actuator which executes high-accuracy control.